1. Field of the Invention
The present invention relates to an electrophotographic image forming technique.
2. Description of the Related Art
Some conventional image forming apparatuses such as a printer, copying machine, and facsimile apparatus transfer, onto conveyed paper, a toner image formed on an image carrier (intermediate transfer belt). To correct the position and color misregistration of an image formed on paper, an image forming apparatus of this type forms a reference image (registration correction pattern) on an intermediate transfer belt, detects it, and calculates the positional error amount (registration error amount). The image forming apparatus corrects color misregistration and the position of an image formed on paper based on the calculation result (image position correction).
To detect the registration correction pattern, a predetermined current is supplied to the light-emitting portion of a correction pattern detection sensor so as to obtain a sufficient amount of light to determine the difference between the registration correction pattern and the surface of the intermediate transfer belt serving as an underlayer. This method may suffer a registration correction pattern detection error if the difference in the amount of reflected light between the underlayer and the registration correction pattern becomes small as a result of, for example, contaminating the intermediate transfer belt or the light-receiving portion, or decreasing the amount of light of the light-emitting portion of the correction pattern detection sensor upon a change over time.
Japanese Patent Laid-Open No. 6-127039 discloses a technique of correcting the amount of irradiating light (intensity) of an optical sensor in order to stabilize an amount of reflected light from the underlayer so as to obtain a constant output value for the surface of an intermediate transfer belt before a print job.
Japanese Patent Laid-Open No. 2002-55506 discloses a method of adjusting the amount of light by detecting the density of a correction pattern.
In light amount adjustment disclosed in Japanese Patent Laid-Open No. 6-127039, the amount of light is adjusted to make an output on the belt always constant. When the intermediate transfer belt gets dirty and the amount of light is increased, an output corresponding to the registration correction pattern also rises. No sufficient difference in amount of reflected light can be ensured between the intermediate transfer belt and the registration correction pattern. The registration correction pattern on the intermediate transfer belt cannot be determined at high precision.
In light amount adjustment disclosed in Japanese Patent Laid-Open No. 2002-55506, no amount of reflected light on the intermediate transfer belt is measured. If, for example, the amount of reflected light from the intermediate transfer belt decreases under the influence of a scratch or the like on the intermediate transfer belt, the correction pattern may be erroneously detected.
When the toner density of the correction pattern is optimum and the intermediate transfer belt is free from any factor such as a scratch which decreases the amount of reflected light, a sufficient difference in amount of reflected light can be ensured between the surface of the intermediate transfer belt and the registration correction pattern.
However, an excessive amount of light is projected by adjustment to increase the amount of irradiating light of the light-emitting portion so as to raise the received amount of light from the surface of the intermediate transfer belt to a given value though the registration correction pattern can be accurately determined. This shortens the service life of the light-emitting portion.
Japanese Patent Laid-Open No. 2006-258906 discloses an arrangement to adjust the density of a formed detection pattern in accordance with aged deterioration of the transfer belt so as to make constant the difference between the light reflectances of the detection pattern and underlying belt (a table holds density data to be adopted). However, this reference does not disclose adjustment of the amount of irradiating light of the sensor.
Japanese Patent Laid-Open No. 2006-251686 discloses an arrangement to determine amounts of emitted light separately when detecting color misregistration and the density by a sensor which has one light-emitting portion and two light-receiving portions and detects specular reflection and diffused reflection. According to this arrangement, amounts of emitted light are calculated from a specular reflection output upon detecting the transfer belt and a diffused reflection output upon detecting the pattern so as to increase the difference between reflection by a transfer belt and that by a pattern. However, this reference does not disclose adjustment of the amount of irradiating light of the sensor.